Recent years have seen the introduction of multi-layer or multi-coat paint systems for metal and plastic substrates, particularly automotive products. Automotive multi-coat systems generally begin with a protective primer coat which is applied by conventional electrocoating processes directly onto a phosphatized or otherwize pretreated automotive body. This electrocoat layer is then typically coated with a primer/surfacer coating, followed by one or more layers of pigmented base coat. While the pigmented base coat may in some cases serve as the finish coat, often a final clear, unpigmented, protective top coat layer is applied over the base coat to provide durability, gloss, depth of color, and distinctness of image to the finished coating. In multi-layer coating systems, the pigmented basecoat is often quite thin, being of the order of 0.5 to 1.0 mils (0.0013 to 0.0025 cm) thickness, while the protective clear top coat is thicker, generally of the order of 2 to 3 mils (0.0051 to 0.0076 cm) thickness.
In recent years, so-called "metallic" effect colors have found wide acceptance among automobile buyers. These metallic effects in automotive finishes are generally achieved by incorporating highly reflective, finely divided particulates into the pigmented base coat of a multi-layer coating system. The particulates are generally aluminum flake, mica particles, or mica particles which have been encapsulated or coated with a metal oxide, typically iron oxide or titanium dioxide. The presence of finely divided reflective particulates in the cured base coat layer produces a metallic sparkle effect which is popular with the automotive consuming public.
While the introduction of "high tech" multi-layer automotive coatings, particularly the metallic color styles, has found wide acceptance in the automotive consuming public, such complex coating systems are not without some associated problems. Not least of these is the problem of producing a total coating system which has the requisite durability and resistance to the damaging effects of weather, ultraviolet light and environmental pollutants.
For example, in a typical "metallic" multilayer coating system, there may be as many as five interlayer interfaces between the substrate car body and the clear top coat: 1) the clear top coat to pigmented basecoat interface; 2) the base coat to metallic or mica flake interface; 3) the basecoat to primer-surfacer interface; 4) the primer-surfacer to electrocoat interface; and 5) the electrocoat to vehicle body interface. The potential exists for delamination or adhesive failure at one or more of these interfaces as a result of weathering or other environmental stress. Sometimes this can be a particular problem in base coat layers which contain metallic flake or mica flake materials. Hydroxyl functions on the metal or mica particle surfaces provide sites for hydrogen bonding of water which finds its way into the base coat as a result of normal weathering processes. This trapping of water can serve to exacerbate the problem of delamination of the base coat layer where the bonding between the metal flake or mica flake particles and the surrounding polymer film matrix is, at best, already weak.
There is thus a need in the automotive coatings art for a means of increasing the weatherability and durability of automotive finishes which contain metallic or mica particulates.